The present invention relates to internal combustion engine control systems and methods. More particularly, the present invention relates to systems and methods for controlling a diesel or Homogenous Charge Compression Ignition (HCCI) Engine.
The defining characteristic of HCCI is that the ignition occurs at several places at a time which makes the fuel/air mixture burn nearly simultaneously. There is no direct initiator of combustion. This makes the process inherently challenging to control. However, with advances in microprocessors and an enhanced understanding of the ignition process, HCCI can be controlled to achieve better emissions along with diesel engine-like efficiency.
In the diesel (stratified charge compression ignition) engine, only air is initially introduced into the combustion chamber, which is then compressed to a relatively high compression ratio (typically between 12 and 22), resulting in a pressure of approximately 30 bar (600 psi). The high compression heats the air to approximately 550° C. (about 1000° F.). At about this moment, with the exact moment determined by the fuel injector driver, fuel is injected directly into the compressed air inside the combustion chamber. The fuel injector atomizes the fuel and distributes the fuel into the chamber.
The temperature of the compressed air vaporizes fuel from the surface of the droplets of injected fuel, the vapor is in turn ignited by the high temperature. The fuel continues to vaporize and burn near the droplet surfaces, until all the fuel in the droplets has been vaporized and then burned. The start of vaporization causes a delay period before ignition. Also, before ignition, some of the vapor premixes with air. When the premixed vapor ignites, the characteristic diesel knocking sound is heard as the igniting premixed vapor causes an abrupt increase in pressure in the chamber above the piston. The rapid increase in pressure then drives the piston downward, supplying power to the crankshaft.
The increase in air pressure and the fact that the diesel engine runs in an effective “unthrottled position” eliminates much of the pumping loss associated with other internal combustion engines, such as Otto cycle gasoline engines. As a result, diesel engines are some of the most efficient engines in use in the world today.
Recent advances in fuel injector technology have aided in the distribution of diesel fuel at injection. Many modern diesel engines have multi stage fuel injection, whereby varying fuel amounts are pulsed into the combustion chambers. Many fuel injectors include Group Hole Injector Nozzles (GHN), or even multiple injector ports. Proper atomization of fuel enables a more even burn, and thus fewer localized rich fuel burn regions. This also leads to fewer regions of high heat combustion, effectively reducing NOx generation and soot formation. However, even given these advances in fuel dispersion, current diesel engines continue to generate unacceptable levels of NOx and particulate matter to adhere to United States and international emissions regulations without the aid of expensive emissions after-treatment mechanisms.
Advances in Diesel Particulate Filters (DPFs) are capable of reducing particulates from the diesel engine. The NOx produced by diesel engines, however, is more problematic.
Spark ignition gasoline engines utilize a 3-way catalytic converter, in the exhaust stream, to reduce NOx into N2 gas and O2. Then slight excess oxygen is used to oxidize un-burnt hydrocarbons and carbon monoxide to CO2 and water. Hence the name, 3 way catalytic converter. The 3-way catalytic converter is capable of these reactions since, in gasoline engines, combustion of the fuel and air mixture vacillates closely about the stoichiometric amounts (substantially in the range of Lambda=0.99 to 1.01), producing periodically a slight excess (for oxidation) or debit of oxygen (for NOx reduction).
Diesel engines, in comparison, are typically operated under lean conditions (approximately Lambda=1.3). That is, there is much more oxygen (from the air) in the combustion chamber than needed for the combustion of the fuel. This results in combustion products that have are low in carbon monoxides and hydrocarbons. As a result, there are not sufficient amounts of reducing agents in normal diesel exhaust to eliminate the NOx using a 3-way catalytic converter.
Currently, in order to introduce a diesel engine which adheres to EPA and other emissions regulations, a reducing agent is typically added to the exhaust system to eliminate excess NOx. In a number of consumer engines, a urea ((NH2)2CO) solution may be injected into the high temperature exhaust flow, at these temperatures, the urea decomposes to ammonia upstream of the catalytic converter. The ammonia may then react, on the catalytic surface, to reduce the NOx, to nitrogen gas (N2) and water. One such urea system, such as the one described, is manufactured by Daimler AG and is known by its trade name “BlueTec”.
While urea introduction is an effective method of eliminating excess NO emissions, there are some substantial drawbacks associated with such a system. First of all, the introduction of a urea solution system is yet another automotive system capable of failure. Also, as a urea solution is a salt solution, deposits of urea can form on the injection nozzle in the exhaust system. These deposits may reduce the system efficiency, or even shut the system down altogether. Thus, urea-based systems may reduce engine reliability.
Secondly, there is an associated up front cost with urea systems. The building and manufacturing of these systems includes additional parts and labor, and these costs are eventually felt by the consumer. Eliminating urea-based systems may reduce vehicle costs.
Lastly, as urea is consumed in the process of eliminating unwanted NOx, there is a need to periodically replace the urea solution in the vehicle. This means there needs to be a separate urea tank, and because consumers are unused to urea resupplying, this is often considered a burden. Additionally, if urea levels are depleted, the engine must shut down to avoid breaking emission regulations. As urea solutions are not necessarily as widely available at gasoline stations, this may pose a large hindrance to the widespread acceptance of urea-based systems.
Other experimental methods of reducing NOx emissions without the need for urea are also being pursued. These systems aim to achieve more stoichiometric burns of the diesel fuel and air within the combustion chamber. The benefit of such systems is that there would be no need for extra NOx reducing systems. Rather, a standard 3-way catalytic converter may be used.
The amount of power produced by a diesel engine is controlled by the amount of fuel injected into the chambers. A typical diesel engine operates over a range of lean to very lean fuel to air ratios. Even at full power most diesel engines operate at a lean ratio of approximately Lambda=1.3 relative to stoichiometric levels of approaching Lambda=1. Thus, at stoichiometric levels (approaching Lambda=1) a diesel engine ends up producing maximum power output but more importantly, the lack of oxygen as one approaches stoichiometric conditions leads increasingly unacceptable levels of conversion of fuel to smoke (diesel particulates). Thus, while it is possible to operate a diesel engine at stoichiometric levels, and then use a basic 3-way catalytic converter (analogous to spark ignited gasoline engines), it is impractical in most commercial applications since the power output of the engine is at a maximum and is not readily adjustable and, for all but the most demanding applications, will produce a large excess of power. Furthermore, the soot (diesel particulate matter) will coat the surface of the catalyst rendering the catalyst inoperative.
In response to this overpower issue at stoichiometric fueling levels, efforts have been made to controllably reduce power output. These efforts rely upon adjusting the incoming levels of oxygen such that less fuel needs to be injected to reach stoichiometric levels. One such approach introduces a throttle plate into the air intake to reduce air flow into the cylinders. While throttling the diesel engine may be effective in adjusting engine output power, this also introduces “pumping loss” in a similar manner to that experienced by traditional gasoline engines. These losses result in a lower efficiency engine, thereby undermining the major advantage of diesel engines.
Another approach devised to reduce power of the diesel engine while achieving stoichiometric operation is to reintroduce exhaust gasses back into the intake manifold (called EGR for “exhaust gas recycle”). Thus, the overall oxygen concentration is diluted in the chamber and less fuel is required to reach stoichiometric levels. Such systems show promise; however, the burn in such systems tends to be less complete than in a mixture containing excess oxygen. Along with increased soot output from this EGR combustion, there is also a drop in fuel efficiency.
Given the current need for more fuel efficiency, and a desire for consumers to have vehicles that are easier to use and maintain, there is a need for improved diesel engine control systems and methods. Such systems and methods may provide enhanced control of diesel engine combustion cycles to generate emissions that are capable of being processed without the need for urea.
In view of the foregoing, systems and methods for improving efficiency diesel engine control are disclosed. The present invention provides a novel system for enabling enhanced control of cylinder fueling and combustion events whereby existing diesel engines may be modified, in a cost effective manner, to satisfy modern emissions standards without the need for a reducing agent such as urea.